RFC 3264 stipulates that an SDP "answer MUST contain exactly the same number of "m=" lines as the offer", and that "existing media streams are removed by creating a new SDP with the port number for that stream set to zero" (i.e. m= lines may be added, but not removed from the SDP). Some UAs don't adhere to this and drop disabled streams in SDP answers or new SDP offers within an existing session (for example when putting the peer on hold). SDP offers or answers missing an m= line will normally cause the snom phone to end the session, unless this setting is enabled.

Note: this setting does not apply in the case when the SDP answer received by the phone contains an additional "m=" line which was not present in the SDP offer sent by phone. In such a case, the audio might fail.